The present invention relates generally to threaded fastener assemblies and, more particularly, to a threaded screw and washer combination which is adapted to be preassembled with the washer axially captivated on the screw shank.
Past fastener assemblies having a washer axially captivated on a screw shank are generally known. Such previous designs are shown, for example, in U.S. Pat. No. 4,732,519 issued to Wagner and in U.S. Pat. No. 5,244,325 issued to Knohl. Wagner shows a threaded screw and washer combination wherein a pair of radially outwardly extending wing protuberances are specially forged on the screw to axially captivate the washer. The washer includes a tubular sleeve having a radially inwardly extending continuous annular bead at its uppermost extremity which is adapted to snap over the wing protuberances of the screw.
In another prior art fastener, detents having radiused or rounded inwardly facing surfaces are provided in the washer below its uppermost end by a tool punching inwardly from the outer surface of the sleeve. These detents block the annular shoulder of a screw, forcibly inserted in the washer, from backing out.
Fastener assemblies of these designs, however, have several inherent problems, including the potential that, in the first case, the ring in the sleeve will deform the wings on the screw when the parts are assembled together. As a result, the mashed or deformed wing protuberances of the screw may not provide sufficient blocking for the washer which could separate axially from the screw.
Knohl discloses a screw and washer combination wherein a spring retainer clip made out of resiliently yieldable material is adapted to be forcibly installed about the screw after the washer and screw have been preassembled. In this way, the retainer clip prevents the washer from slipping off the screw. With this design, however, additional manufacturing and assembly of the retainer clip are required before the fastener assembly can be preassembled.
In the other prior assembly, the radiused detents were found not to be consistently effective at preventing screw withdrawal. The radius provided a means by which "jiggling" or tilting manipulation of the screw could wedge the shoulder between the detents and it could "walk" out of the sleeve. Also, on assembly, the radiused or pointed detents could deform the softer screw shoulder surface, creating reliefs in the screw shoulder through which the detents could retract, permitting screw and washer separation. In other words, it is desirable to provide a significant or substantial abutting overlap of detent to shoulder after assembly. The radiused detents, however, tend to deform the screw shoulder permanently upon assembly, reducing any subsequent overlap when the detents later align on the shoulder with deformed areas and leading to a greater likelihood of separation than is desired.
Accordingly, there is a need for a new fastener assembly having a preassembled screw and washer combination which provides for a reliable interference fit between the screw and washer such that the fastener assembly consistently does not separate during handling. It is also desirable to reduce the number of parts and special manufacturing and assembly steps in the manufacture of screw and washer assemblies, as compared to certain prior assemblies.